`Ta có : (x^2+4x+3)(x^2+6x+8)-8` $\\$ ` = (x+1)(x+3)(x+2)(x+4)-8` $\\$ ` = (x+1)(x+4)(x+3)(x+2)-8` $\\$ ` = (x^{2} + 5x +4 )(x^{2} + 5x + 6)-8` $\\$ `Đặt : x^{2} + 5x + 5 = y` $\\$ ` =>x^{2} + 5x +4 = y-1` $\\$ ` => x^{2} + 5x +6 = y+1` $\\$ ` => (x^{2} + 5x +4 )(x^{2} + 5x + 6)-8 = (y-1)(y+1)-8 = y^{2} -1 - 8 = y^{2} -9` $\\$ ` = (y-3)(y+3) = (x^{2} + 5x + 5 - 3)(x^{2} + 5x + 5+3)` $\\$ `= (x^{2} + 5x + 2)(x^{2} + 5x +8)`
Chúc bạn học tốt.
